The day after he swam his last race as a competitive swimmer at the London Olympics, Michael Phelps got up early.

It was force of habit. He didn’t have a race that day, or even a scheduled training session with his coach of 16 years, Bob Bowman. For two decades, the 27-year-old Baltimorean was up at dawn for his first of two daily swims, clicking off the millions of laps of training that earned him the title of nothing less than the greatest Olympian of all time. With an unprecedented 22 Olympic medals, 18 of them gold, he is now the most decorated Olympian ever,

“There will be no more staring at the black line [at the bottom of the pool] for four hours every day,” he said that morning in London. “This is the first day of my retirement. The first day of my new life.”

And just what will he do with the rest of his life? Travel and golf, he told the reporters gathered to document the first day of the rest of his life.

But who is he kidding? The pool and Phelps will be forever linked, forged in an unbreakable bond as interlocked as the five Olympic rings. Like the whiff of chlorine that trails every water baby, he won’t be able to shed the sport of swimming so easily.

Nor does he want to. While always cagey about disclosing his personal goals during his competitive career, Phelps has said that one legacy he hopes to leave behind is a sport that is stronger, bigger and higher profile than when he entered it. “I want to grow the sport of swimming,” is his refrain. His agent, Peter Carlisle of Octagon, says that desire was born early, at his first meeting with the then-15 year old Phelps, who was shopping for representation. Ensconced in a lawyer’s office, Carlisle and his team were talking to Phelps’ team as if the teen wasn’t in the room. During the lunch break, Carlisle prodded the boy to speak up. Nervous, Phelps dropped his plate of food but had his answer: he wanted to see swimming on SportsCenter.

“The problem was, at that time, Michael had this abstract potential, but he didn’t have a platform,” says Carlisle. “He was the youngest swimmer to break a world record but if a tree falls in the woods and there’s no one there to hear it, does it make a sound? The general public had no idea about Michael or about swimming. That’s where we were in 2002.”

Fast forward a decade, and swimming does pop up on SportsCenter, world championships earn coverage by ESPN, and at the past two Olympic Games, it occupied the place of honor during NBC’s prime time coverage of events. There’s no question that Phelps and his journey, first to surpass Mark Spitz’s 36-year old record of most (seven) Olympic golds in a single Games, and then to best Russian gymnast Larissa Latynina’s 48-year old record of most medals earned by an Olympian, were a big part of that. (And that his brief falls from grace, including a DUI arrest and a three-month suspension from USA Swimming in 2009 after a photo of him inhaling from a marijuana pipe surfaced, only made his story more human — and newsworthy.) In Athens in 2004, in Beijing in 2008 and in London in 2012, the big question in the aquatic center and in the minds of the billion of viewers who tuned in to watch swimming worldwide was: ‘could Phelps do it?,’ would Phelps do it?’

“There is no question that Michael brings instant rock star status to any swim meet,” says Evan Morgenstein, president and CEO of PMG Sports, an agency that represents Olympic athletes. Agrees Carlisle: “Michael’s journey was unique enough to draw in and keep an audience. He kept the general public interested, and helped increase the relevance of swimming for them.”

Since 2001, USA Swimming membership has increased by 25%, and more of those athletes are sticking with the sport (70% in 2010 compared to 65% in 2001). Clearly not all of that interest can be directly attributed to Phelps, but after Beijing, the organization saw its highest increase in year-round memberships in 23 years. At the last swim of his career in London, fans poured into the Aquatic Center from around the globe.

“He is a legend; it’s unbelievable what he has achieved,” Victor Kolar, a gym teacher from the Czech Republic who came to watch Phelps with his wife and two daughters marveled. “We want to be past of his last celebration.” For his daughters, aged 12 and 7, Kolar hopes Phelps will serve as a role model. “He showed great determination to succeed, and it’s good to learn from him, learn how to win, and learn how to be resilient,” he said.

Still, as challenging as his remarkable feats in the pool were, Phelps faces an even bigger obstacle now that he won’t be diving into races any more. Granted, in some ways his dream of building up the sport of swimming becomes easier, as he sheds his rigorous training schedule and becomes freer to become an ambassador for the sport. (An offhand comment that he has always wanted to dive with great whites elicited a flood of offers from dive outfits and even the Discovery Channel, which plans to document a trip to South Africa in which he cage dives to get up close and personal with the beasts to air during, natch, Shark Week.)

But will people still watch swimmers splashing anonymously in a pool covered in goggles and a cap if Phelps is not among them? “Everybody in the swim industry, whether a coach, a parent to a swimmer or a swimmer, is asking the same thing,” says Morgenstein, who represents gymnast Nastia Liukin and swimmer Dara Torres. “They know that their future in some way, shape or form is dependent on how popular the sport of swimming is. If the sport stays popular because Michael Phelps stays involved with it, then he has created a legacy beyond his accomplishments.”

And that legacy goes both ways. “You can’t just take Michael as a totally independent, disconnected character and create a brand around him as though he doesn’t depend on the sport, or the sport doesn’t depend on him,” says Carlisle. “He needs the platform of swimming; he needs the Olympics and the Olympic partners. Otherwise the moment he is done competing, he’s not relevant any more.”

And the reality is that swimming, and swimmers, jostle in a crowded US sports market that rewards sports with well-established, high profile professional leagues. Swimming comes around once every four years, and while there are no shortage of swim programs at the youth and college level, it lacks a professional circuit of events that can sustain a swimmer who wants to turn his laps in the pool into a living. Sure Phelps was able to break through and get his story covered on ESPN (and on the cover of TIME), and become a regular spokesperson for sponsors like Subway (even in non-Olympic years), and, more recently, Louis Vuitton. But it literally took an historic feat of Olympic proportions for him to earn that status.

When I first spoke to Phelps before the Athens Games in 2004, he was frustrated by the corporate world’s lack of interest in swimming. “Look at the new Sprite commercial, it’s LeBron [James],” he said. “You don’t see a swimmer doing a Sprite commercial.” And the sad truth is, you still don’t. “Swimming is always going to be an Olympic sport,” says Morgenstein. But he acknowledges that Phelps has a unique opportunity to spin the sport’s cycle beyond its usual four-year revolution. “Millions of kids swim during the summer,” he says. “It would seem that there is enough momentum associated with Michael that he can motivate kids above and beyond his swim centers.”

Whether that is in the form of a professional swimming league that allows Olympic-eligible athletes to earn winnings depending on their point total in a series of meets, or whether it means creating a youth league modeled after Little League that gives promising swimmers more support throughout their career, will be up to Phelps. His stature in the sport is such that he might be only one to seed such a change. “Michael and I are putting together very unique ideas for some events that are media and spectator friendly that haven’t been done before,” says Bowman, in a hint of where we might see Phelps next.

For now, however, he says Phelps is focusing on expanding his swim schools, which are part of the Michael Phelps Foundation, begun in 2008 with the $1 million bonus check he received from Speedo for breaking Spitz’s record. Phelps’ im program, which is taught at the schools, begins with water safety for children who have never been in a pool and extends to stroke training. So far, it’s part of 28 Boys and Girls Clubs in the U.S. and Phelps hopes to expand the program to all 200 of the Clubs with a pool and provide transportation for youngsters without access to pools so they can participate as well. Growing the sport will mean nurturing its grass roots, and not simply lending his presence when it’s convenient, a commitment that Phelps seems to have already made.

“I want to take swimming to a new level,” he said before leaving London. “It hasn’t reached the peak I want it to reach. That’s tough for swimming in the U.S. because we have so many sports, so many professional sports in the spotlight. Hopefully we can catch up and be on the same level. I’ve been able to see the sport change so much in the last four years, it’s shocking.”

It will take more of that kind of shock, and the constant reminders of the awe that Phelps has inspired during his career, to draw both new swimmers and corporate sponsors into the sport. “We will certainly miss the thrill of watching him compete,” says Bowman, “but he will still have a platform, and more people involved in swimming can only help to grow the sport.” And maybe one day put a swimmer in a Sprite commercial.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.